Laser communication units (also: laser communication terminal, LCT) need to keep the orientation of the transmitted beam in relation to the reception direction or in relation to the position of the remote station stable in a precise manner over some time (from several minutes to days). This is necessary in order to ensure a high level of signal quality for the received signal at the remote station.
The divergence of optical laser communication systems is normally a few rad, and a misorientation of the transmitted beam in relation to the received beam of a few rad therefore quickly results in severe losses on the transmission link. Additionally, moving laser communication systems require the direction of the transmission layer to be changed slightly, in comparison with the direction of the received light (a few μrad to 100 μrad), in order to compensate for effects of propagation delays. This angle can be referred to as a compensation angle or lead angle. This angle is usually known, but is actively regulated (with a small bandwidth) for longer transmission links so that no additional losses arise on the transmission link. In general, the transmission direction differs from the reception direction over time (several minutes) as a result of thermomechanical effects, even if they have been calibrated using suitable means before connection setup.
DE 10 2012 012 898 A1 and EP 2 680 024 A3 describe a system and a method for determining the position of a communication platform in the form of a satellite.